Electrolyte Decomposition and Electrode Thickness Changes in Li-S Cells with Lithium Metal Anodes, Prelithiated Silicon Anodes and Hard Carbon Anodes

Abstract

Li-S cells can have high gravimetric energy densities above 300 Wh kg−1 when the electrodes and cell components are optimized. Low electrolyte/sulfur mass ratios or more generallly, the relative amount of electrolyte in a Li-S cell have an especially high impact on the achievable gravimetric energy density. A negative side effect of low electrolyte/sulfur ratios are low cycle numbers due to electrolyte decomposition and the possibility that electrolyte becomes inaccessible at the lithium metal anode when the lithium becomes more and more porous during cycling. Electrode thickness measurements were performed during cycling for various cell chemistries such as lithium-sulfur (Li-S) with different cathode sulfur loadings and porosities, lithium-hard carbon (Li-HC), lithium-silicon (Li-Si), prelithiated HC-sulfur (LiHC-S), prelithiated Si-sulfur (LiSi-S) and Li-ion. The thickness measurements provided information about mechanical stress and irreversible thickness changes. The thickness measurements also helped to explain different electrolyte decomposition behavior and they can be used to discuss the impact of thickness changes on gas analysis. The electrolyte decomposition of the Li-S standard electrolyte based on lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in dimethoxyethane (DME): dioxolane (DIOX) with LiNO3 was examined by online mass spectrometry (MS) within Li-S, Li-HC, Li-Si, LiSi-S and LiHC-S cells. Several electrolyte decomposition products were verified by post-mortem gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS) of Li-S cells with cycled electrolyte.